1. Field of the Invention
The present invention relates to a tone signal detection apparatus for detecting a tone signal of a specific frequency.
2. Description of the Related Art
With communication apparatuses such as a modem and a facsimile apparatus, various tone signals are detected in advance of starting communications after circuit connection. For data communications on a telephone network, an international standard has been agreed to as ITU-T recommendation series V, wherein V.8 provides for a standard starting procedure for a data transmission session.
According to the V.8 procedure, a terminal on the call arriving side (receiving terminal) transmits an ANSam signal (deformed response tone) to a terminal on the call originating side (calling terminal) as soon as a circuit connection is established. When the calling terminal receives the ANSam signal, the calling terminal transmits a CM (Call Menu) signal that contains information about functions of the calling terminal. When the CM signal is received during ANSam signal transmission, the receiving terminal transmits a JM (Joint Menu) signal that contains information about functions of the receiving terminal.
By exchanging the signals as described above, the V.8 procedure moves on to a next phase of the communication such as V.29. For example, if the calling terminal desires to start facsimile communications, and if the receiving terminal is capable of the facsimile communications, communications that follow a TC protocol and ITU-T T.30 recommendation are carried out.
The ANSam signal is made of a 2100 Hz sinusoidal wave, the phase of which sinusoidal wave is reversed at 450 ms intervals, and the sinusoidal wave is amplitude-modulated by 15 Hz. On the other hand, a low-speed receiving terminal discernment (CED) is a 2100 Hz sinusoidal wave.
The calling terminal determines that an ANSam signal is received if both the 2100 Hz signal and the 15 Hz signal are detected, and determines that a CED signal is received if only the 2100 Hz signal is detected.
According to Patent Reference 1, the 2100 Hz signal is detected by a tone detector that is firmware including a DSP (Digital Signal Processor).
A down sampling circuit is provided in front of the tone detector, wherein an input signal is down-sampled, and the down-sampled signal is provided to the tone detector so that a signal of a specific frequency is detected.
For example, the ANSam signal is detected by an envelope of the amplitude-modulated 2100 Hz tone.
[Patent Reference 1] JPA 2002-290505